Full Disclosure: Chrome 11 Anti-XSS ByPass

Chrome 11 Anti-XSS ByPass

From: Manuel Fernández Fernández <mfernandez () informatica64 com>
Date: Wed, 25 May 2011 17:50:34 +0200

Hello,

During the creation of a hacking challenge about XSS we had to figure out how to bypass the new AntiXSS filter in 
Google Chrome. It was included in the latest release and we were in the middle of a hacking challenge about XSS and 
Sesion Fixation. We were thinking about to change the rules of the game, but, we managed to bypass the filter in an 
easy way, so we didn´t change it and players were also able to discover it. This is the "how":

When a string that can execute any kind of javascript which it is injected, like could be <img src=”noexist” 
onerror=”alert();”/>, Google Chrome fix the source code in runtime, replacing the original string by the following, 
<img src="noexist" onerror="">, so the injected javascript code is missed and it’s not possible to execute any attack.

Chrome also has a function to fix or rewrite HTML source code that isn’t well-formed. If a website that contains a tag 
like ‘’, it will try to rewrite the source code to construct a proper HTML code. For example, if we introduce a string 
like <img src="noexist" onerror=alert, Chrome will try to close the img tag with the following closing tag, in this 
case <html>.
<img src="noexist" onerror="alert!!!</html">

It can give to an attacker a vector to bypass the Anti-XSS filter, due to it applies first the filter and then checks 
whether the HTML code is well-formed or not, modifying it if needed..

So, it’s possible to create a non-well-formed tag, that after being rewritten, it will be a properly XSS.
An example of it may be this one: <img src="noexist" onerror=alert();//, due that it would be rewrote as <img 
src="noexist" onerror="alert();//!!!&lt;/html">, bypassing the AntiXSS filter.

Best Regards,

Manu "The Sur" and Rodolfo Bordon from Informatica64.com

Manuel Fernández
Informática 64
Dep. Desarrollo y Seguridad
C/ Juan Ramón Jiménez, 8 Bajo Post. Local
28932 Móstoles - Madrid
Telf. 91 146 20 00 – Fax: 91 146 20 03
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